By your diagram,it's obvious that you understand the concept.I myself might like to try a cool tube,but before I go sticking one over the top of 5 chroming at the mouth females,2 feet away from their beautiful HEADS,I'm gonna find out from someone if theirs is still working.I think tempered glass might work...Where do you find 4" pyrex tubes?
b.s.
ps..you don't want to put an exaust fan @ the bottom of your enclosure,it will only suck the cool air out,and pull heat downward against the nature of the force.